Field of the Invention
This invention relates to Radio Frequency Communication transceivers and in particular to RF filters with reduced intermodulation distortion characteristics.
Description of Related Art
A typical wireless communication system, such as cellular transceiver, includes uplink and downlink channels separated in frequency. Such communication systems use filters to route, combine, and/or separate signals at different frequencies, to prevent interfering with other channels or systems, and/or to prevent being interfered with by other channels or systems.
One type of filter used in such communication systems is constructed with coaxial cavity resonators, sometimes referred to as combline or interdigital resonators. These resonators typically consist of a metal outer conductor or cavity with a metal inner conductor. The inner conductor is electrically short circuited to the outer conductor at one end and open circuited at the other end. When an electromagnetic wave is coupled to this structure, the wave propagates along its length until it encounters the short circuit and is reflected back. This reflection causes a standing wave to be generated when the length of the inner conductor is approximately ¼ wave length long relative to the frequency of the coupled wave. Shorter lengths can also be used by capacitively loading the open circuit end. This standing wave can then be further coupled to adjacent resonators, allowing waves at specific frequencies to propagate while rejecting waves at other frequencies.
However, coaxial cavity resonators can cause signal corruption. Signal corruption can occur when intermodulation Distortion (IMD) generated by the uplink or downlink signals fall unintentionally into the downlink or uplink frequency band, respectively. IMD in filters can create the very interference they are supposed to be preventing.
As such there is a need to enhance the performance of such coaxial cavity resonators employed in wireless base stations and to specifically reduce or preferably eliminate intermodulation distortion.